The utilization of a non-coherent doppler processor in a pulsed energy system, such as an airborne radar system for the detection of moving targets, has been described in a copending application Ser. No. 391,073 for an AMTI Radar System, filed Aug. 18, 1964, now U.S. Pat. No. 3,408,647, by F. J. Dynan et al, assignors to North American Aviation, Inc., assignee of the subject invention. The display of a substantially clutter-free moving target by means employing such non-coherent doppler processor, relies on the presence of a substantial clutter content in the received signals, which gives rise to a clutter-referenced moving target signal which may be spectrally distinguished from the d-c, or low frequency, energy of the detected clutter content. For such purpose, a high-pass analog-doppler filter is employed, having an upper corner frequency (or cut-off frequency) not exceeding one-half the pulse repetition rate of the pulsed energy system employing such device.
Such prior-art arrangement has several inherent disadvantages. First, the doppler processor requires a substantial storage medium for storing a plurality of successive range trace signals (several hundred or more) for creating a data matrix of range-bin versus pulsed interval, in order that the data in a given bin may be sequentially scanned (in the same sequence as the occurrence of the pulse intervals in which the data were recorded), for reconstructing the clutter-referenced moving target spectra. Secondly, the fixed bandpass of the analog doppler filter means may respond to the clutter spectra, as the bandwidth of the clutter spectra spreads with increases in platform velocity and with changes in look-angle (in a scanning system); while a moving target may not be detected, even though spectrally distinguishable from the clutter and below the fixed bandpass of the high-pass doppler filter. The blind speed effects imposed by the use of fixed bandpass filters may be relieved to some extent by the use of filter chains comprising severally switchable narrow-bandpass filters for covering a doppler bandpass of interest, as described in the above-noted copending application Ser. No. 391,073. However, such switching of discrete portions of the bandwidth merely tends to reduce, rather than avoid such effects.
Another prior art method of cancelling the d-c or time-averaged signal level of a clutter-referenced range trace signal, involves the use of delay line cancellers, such as double-delay line cancellers, canonical-configuration comb filters and others, as described more fully in Chapter 4 of "Introduction to Radar Systems" by Skolnik, published by McGraw-Hill (1962). A common disadvantage of such delay line techniques is the reliance upon a very precise, constant system pulse repetion frequency (PRF). Also, such delay line elements require accurate calibration among them, and tend to be temperature sensitive, thereby degrading the performance results obtained by such techniques. In other words, such techniques impose serious constraints on both system and component design.
A further prior art method of doppler-processing a clutter-referenced range trace signal is the range-bin sampling of the range-trace signal by a plurality of range-gated clutter-rejection filters and processors, as shown, for example, in U.S. Pat. No. 2,600,193 to Bell and at FIG. 4.41 of "Introduction to Radar Systems" by Skolnik, published by McGraw-Hill (1962). In such arrangement, the amplitude of a sampled range-bin is stored by a gated boxcar generator circuit for processing by a bandpass limited clutter-rejection filter, and the doppler modulation frequency component is then detected, integrated and thresholded. This type of processing has several inherent disadvantages. First, larger storage capacity and therefore larger storage elements are required, limiting the applicability of micro miniaturization techniques to such processing. Also, extremely low impedances are required for the boxcar detectors in order to effect rapid charging/discharging of the storage elements, while high output impedances are required for impedance isolation in the signal storage function. In other words, considerable design complexity results in seeking to achieve satisfactory series switching of the boxcar device.